1. Field of the Invention
This invention relates in general to heater bars for reflow soldering and more particularly to such heater bars energized by current pulses.
2. Description of Related Art
Welding, as it will be used herein, includes various processes for bonding metals, including fusion, brazing and, most particularly, reflow soldering. Reflow soldering refers to that method of bonding in which metal parts are precoated with solder and are heated during bonding until the solder melts or "reflows." Once this has occurred, heat is removed, the solder solidifies and the parts have been bonded. Reflow soldering may be either continuous or pulse heated. In the continuous mode a holding member is interposed between the metals that are to be bonded and the heating element. The intermediate holding member secures the parts after they have been heated and after the heating element has been withdrawn, so that the reflowed solder may cool without the parts being disturbed by withdrawal of the heating element. With pulsed heat reflow soldering, the heating element is pressed directly against the parts to be bonded, serving both as a source of heat and as the means for holding the parts together until the reflowed solder has solidified. In a typical application the heating element is in the form of an electrically and thermally conductive heater bar mounted on a welding head having a series of spaced apart electrical terminals. The heater bar, whose resistance is relatively high, is resistance heated by the current which flows through it. It includes a bar-shaped transverse member having a front working surface which engages the parts to be soldered and a rear surface. Current is supplied to the transverse member through a plurality of spaced apart members, each including at one end a connecting leg extending from the rear surface of the transverse member at an angle and terminating at its opposite end in a contact pad. The heater bar is mounted on the welding head by engagement of respective ones of the mounting pads with corresponding ones of the welding heads' electrical terminals.
Two problems have been experienced with the above type of heater bar: Expansion, in general, and deformation as a result of uneven expansion.
Expansion of the heater bar is a particular problem with pulse heating where the heater bar contacts the workpiece directly. Expansion of the heater bar during the bonding process while it is being pressed against the workpiece and while heat flows from the heater bar into the workpiece can easily cause bonding pads on the workpiece against which the heater bar is being pressed to be slid from their proper positions, causing permanent damage.
Uneven deformation, on the other hand, damages the heater bar itself. It is caused by the fact that the mounting pads are solidly anchored in the welding head, whereas the transverse member of the heater bar is free to expand and contract. After a number of heating cycles this uneven expansion causes permanent deformation of the heater bar, manifested in a permanent bend in that transverse member. This not only destroys the flatness of the heater bar's working surface, it also makes the heater bar difficult to remount on the welding head once it has been removed, because the effect of the bend in the transverse member is to cause the spaced-apart members, which include the connecting legs, to become tilted relative to their original orientation.
Accordingly, it is the principal object of the present invention to so modify heater bars currently in use as to significantly reduce their tendency to expand when heated. In accordance with the invention, the above and other objects of the invention are achieved by a heater bar whose transverse member is slotted along planes which intersect at least some of its connecting legs. By introducing slots in the transverse member, the cumulative effect of the expansion of individual portions which lie between adjacent slots is largely eliminated, so that the total expansion of the transverse member is greatly reduced. Preferably, each slot extends entirely through the transverse member and into one of the connecting legs. The connecting legs into which the slots extend may extend all the way to the mounting pads, whose cross sectional area will usually be substantially greater than that of the connecting legs which they terminate. In that event it is preferred that the slots extend not only into the connecting legs but substantially all the way to the mounting pads, so that the connecting legs are split along their entire length.
In one preferred embodiment, where the heater bar is machined from flat stock so that it is defined by a pair of surfaces lying in spaced-apart parallel planes, it is preferred that the slots extend transversely, i.e. lie in planes which intersect the first and second planes substantially orthogonally. However, in accordance with another preferred embodiment of the invention whose purpose will become clearer as this description proceeds, the slots are slanted, i.e. so as to lie in planes which intersect the first and second planes defining the heater bar at an acute angle, preferably substantially diagonally through the connecting legs.
In some applications it is necessary to extend the distance between the transverse member of the heater bar and its series of mounting pads in order to reach recessed parts that are to be bonded. In such applications the spaced-apart members, which include at one end the contact pads and at the opposite end the connecting legs, include, in addition, an intermediate portion whose cross-sectional area is greater than that of the connecting leg in order to provide an adequate path for the current that must be conducted from the mounting pad to the connecting leg. Such an intermediate portion may be simply a linear extension of the mounting pad, so that the mounting pad is attached at its terminal end to the welding head and extends away from and beyond the welding head toward the transverse member, terminating in a connecting leg whose cross-sectional area is substantially smaller than that of the mounting pad. With this type of heater bar it is preferred that the slot extend through the transverse member into the connecting leg up to the transition between the connecting leg and the extended mounting pad. By so terminating the slots, the connecting legs, which because of their smaller cross-sectional area tend to get much hotter than the intermediate portion which connects them with the mounting pads, are free to spread as the transverse member expands.